With the Olympics in Rio underway I thought it would be cool to explore some of the physiology of the most legendary athletes. Usain Bolt certainly fits into this category. He’s aiming for 3 gold medals in 3 consecutive Olympics. Now, while you might normally think that his performance is powered by his muscles (and it is), there is one deeper level of physiology we can explore that will help you to appreciate how incredible his performances are. Let’s take a look at the what happens to the nervous system during the 100 m dash.

Let’s look at Usain Bolt’s world record 9.58-second 100-metre dash. Exploring “the start” is fascinating when we consider the lighting storm of electrical activity involved. There are two critical stages of the run itself: the acceleration phase and the speed maintenance phase and that is what we will be exploring in this post.

THE ACCELERATION PHASE

A special type of movement takes advantage of the nervous system to supercharge muscles. Each time that Usain Bolt’s foot struck the ground during the acceleration phase of his 100- metre dash, he took advantage of this reaction, called plyometrics. You’ve probably experienced the plyometric reaction in your doctor’s office during your annual physical exam. Remember when your doctor takes out his little mallet and taps your patellar tendon right below the knee? Your quadriceps muscle and the tendons that connect the muscle to bone stretch very quickly, thanks to this motion. The sudden lengthening of your muscle sets off what is called the stretch reflex. That reflex is modulated by two special nerve fibres—muscle spindles and golgi tendon organs (GTO).

Muscle spindles are sensory receptors that wrap around muscle fibres and detect changes in the length of the muscle. So if you slowly move your muscle through a range of motion, the mus- cle spindles will not sense any danger and will remain “silent.” If, however, the muscle tendon complex lengthens quickly, then the muscle spindles fire signals back to the central
nervous system. These signals activate alpha motoneurons, leading back to the muscle that’s being lengthened. Similarly, golgi tendon organs, which are located in the area where the ten- dons link to the muscle, are sensitive to the force that’s developed. Both muscle spindles and golgi tendon organs serve as protection mechanisms that likely evolved to prevent muscles and tendons from being torn or damaged. When the signal reaches the spinal cord, immediate signals are sent back to the muscle along the neurons that are connected to muscles—the process takes less than 0.02 seconds. The muscle responds by contracting, and that’s why you see your leg kick up right after your doctor hits your patellar tendon with that little mallet. The really cool aspect of this response is that it’s one of the few movements happening in the human body that totally bypasses the brain. Sending the signal back to the brain would take too long to effectively protect the muscle.

Each time Usain Bolt’s toes and the ball of his foot hit the ground, his Achilles tendon and calf muscle fibres were placed on stretch very quickly. This position would have stretched the elastic components of the muscle tissue (connective tissue and fascia, for example). But it may also have quickly stretched the muscle fibres and activated the plyometric reflex, thereby increasing the force of the muscle contraction that then propelled him down the track.

The plyometric reaction can be seen in many sports. Basketball players demonstrate it when they explode off the ground before dunking a ball. Among track and field athletes, it appears the moment before takeoff on high jump. It’s seen in mogul skiing and in the explosive movements of gymnastics. In these critical moments, muscles are loaded very quickly during an eccentric contraction (when the muscle is contracting and lengthening at the same time); then, using the plyometric contract in addition to powerful motor signals from the brain, they contract concentrically very quickly to generate the power that the athletes need to execute their moves.

MAINTENANCE OF MAXIMUM-VELOCITY PHASE—FIGHTING FATIGUE FOR THE WIN

The critical phase of the 100-metre dash is the maintenance of speed phase when athletes fight metabolic and neural fatigue. Most if not all athletes actually decelerate during the 100-metre dash, even though it lasts only 9.58+ seconds and the athlete who wins is usually the one who decelerates the least. Inside the body, it comes down to the athlete with the best resistance to fatigue—and the nervous system, just like muscle, is subject to fatigue factors. During exercise the brain sends signals to the muscles, directing them to contract and helping us to perform. In addition to those nerves which take information to the muscles that initiate and control movement, there are many nerves that collect information about the state of the muscles, blood vessels and other structures and relay that information back to the brain. This feed-forward and feedback system serves to regulate and control exercise intensity. Some scientists believe that the communication between the brain and the body is what ultimately limits exercise performance. They have termed this process the central fatigue hypothesis. In this case, the word “central” refers to the brain as the master controller for exercise intensity in the body.

So simply put in the final stages of the 100 m race there is a war going on inside the body between the brain and the muscles. Now that you know the science I hope that you can appreciate this incredible event event more!